In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘signal beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded signal beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a single or few 2-dimensional layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the signal beam by the spatial light modulator (SLM) and detected with a detector array.
In an apparatus for reading from and/or writing to disk-type holographic storage media the objective lens is mounted on an actuator. The objective lens, therefore, moves relative to the remaining optical components, e.g. for tracking or to keep the objective lens on a fixed position relative to the rotating disk surface during recording. The shift of the objective lens causes a strong distortion of the hologram during recording, and also a shift of the pixels on the detector array, because the remaining optical system does not move. Consequently, the negative effects of the movement of the objective lens have to be reduced to a minimum. Similar problems arise for other types of optical storage media, especially for high-density optical storage media.
U.S. Pat. No. 5,828,482 discloses a beam shifting element, which achieves a beam shift by tilting a parallel plate. However, for an apparatus for reading from and/or writing to holographic storage media a thick glass plate is required to achieve the necessary lateral shifts of about 100 μm. This makes it difficult to achieve compensation at a high frequency, as the required forces are relatively high. The tilting of a parallel plate is, therefore, mainly a solution for a quasi-static system.